Optical Discs

CD Compact Disk

This is often called CD-ROM. The disc is the same size as an audio CD and it is read in the same way. Data cannot be changed (hence the ROM). Access is slower than a hard disc (pack or Winchester) because the spin speed is slower. A CD-ROM can store about 650 megabytes of data. They are used in situations where the data does not age quickly. Examples include encyclopedias, large catalogues and telephone directories. Software is nearly always supplied on CD-ROM. This is because a modern program may require more than 10 floppies to provide the data for installation. In this situation a CD-ROM becomes cost effective.

WORM optical disks (Write Once Read Many) sometimes called CDRW allow the user to write onto the surface of a CD-ROM. This can only be done once since the surface is permanently changed during the writing process. The resulting disc can be read by a normal CD ROM drive. This is useful for archiving data or for small-scale CD ROM production. Windows XP allows this process to occur fairly seamlessly. Before the school had XP on all machines students often brought in such CDs and didn't understand why the older operating systems the school had couldn't access them.

© LEV